radv: Revert divisor = 0 case for vertex attribute extension.

Seems like DXVK depends on that and it might get reverted
upstream. Since apps are not supposed to use 0 in v2 anyway,
we should be safe implementing the old behavior there.

Fixes: 66e12451 "radv: Update to new VK_EXT_vertex_attribute_divisor to version 2."
CC: 18.2 <mesa-stable@lists.freedesktop.org>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
(cherry picked from commit 011a8116)